1. Field of the Invention
This invention relates to an extrusion head for blow molded extruded plastic containers.
More specifically, this invention is related to an extrusion head with a ring gap nozzle having a mandrel and a ring-shaped nozzle body, an elastically deformable sleeve, and setting devices for radially deforming the elastic sleeve, whereby the sleeve defines a nozzle gap. The geometry of the nozzle gap is defined by deforming the elastic sleeve during the course of extrusion of a hose-like pre-shaped blank.
2. The Prior Art
The prior art discloses a multitude of extrusion heads disposed next to each other as a multiple blow head [see xe2x80x9cPlastverarbeiter Publicationxe2x80x9d 44 (1993), No. 2, page 28]. With such a design, it is possible to mechanically couple the setting devices associated with the extrusion heads with each other in order to reduce the number of required, expensive drives.
With a design known from FIG. 6 of German Patent DE-C 28 23 999, the lower edge of the sleeve is supported on the body of the nozzle. This design has been successfully employed in industries for producing pre-shaped blanks having large diameters. However, problems may arise in the production of hose-like, pre-shaped blanks with a small diameter. With a ring gap nozzle with a smaller diameter, it is necessary to use a very thin-walled sleeve, so that the sleeve can be deformed as required for providing the annular gap with the desired profile. The support surface on the lower edge of the sleeve is correspondingly small. Thus, when the mandrel is extended in combination with a deformation of the sleeve, there is the risk that the melt, when exiting from the gap of the nozzle, accumulates on circumferential sections of the holding ring belonging to the body of the nozzle. This leads to markings and differences in the wall thickness in the hose, resulting in a loss of quality of the plastic container that is blow molded from the hose. This accumulation in the hose is also often connected with adhesion problems causing production trouble.
With a design shown in FIG. 7 of German Patent DE-C 28 23 999, the sleeve, at its end on the inlet side, has a dimensionally stable collar, with a flange surface extending circumferentially in the form of a ring. The flange surface in supported in the body of the nozzle over its entire surface area. The axial forces exerted on the sleeve by the flow of the melt are fed into the body of the nozzle via the collar.
The invention further provides an improved extruder head with the features described above so that extensive and largely wear-free deformations and displacements are possible at the end of the sleeve located on the hose outlet side, whereby accumulation of the melt in the body of the nozzle is extruded as such melt exits from the outlet gap of the nozzle.
The problem is solved according to the invention in that the sleeve is axially supported only on segment-shaped surfaces. The sleeve can be elastically deformed to alter the geometry of the gap to generate different thicknesses of the plastic container.